This invention relates generally to the production of high purity hydroxylammonium nitrate and, more specifically, to a process for the production of concentrated hydroxylammonium nitrate from hydroxylamine and nitric acid.
Hydroxylamine nitrate has been produced by several processes utilizing hydroxylamine sulfate and converting it to the end product by processes such as electrodialysis or a cation-exchange process. Some processes produce aqueous hydroxylamine from hydroxylammonium sulfate. However, heretofore none have been found which produce the aqueous hydroxylamine salts by neutralization of the corresponding acid, without causing spontaneous decomposition upon addition of the concentrated acid. Conversely, addition of the hydroxylamine to nitric acid causes spontaneous decomposition of the product HAN, even when the nitric acid has been diluted to less than about 50% by weight.
Hydroxylamine nitrate has several commercial applications, such as in the purification of plutonium metal, as one component of a liquid propellant, and as a reducing agent in photographic applications. In some of these applications a highly purified form of the compound is required, especially when it is to be employed in propellant formulations where the hydroxylammonium nitrate (HAN) solution is stable in an aqueous solution, but must be completely free of transition metal elements, such as iron and copper.
Previous electrolytic processes have electrolyzed nitric acid solutions containing mineral acids such as sulfuric acid or hydrochloric acid to form hydroxylamine salts of these acids. The processes were carried out in an electrolytic cell having high hydrogen overvoltage cathodes, such as mercury or an alkali metal amalgam, with a diaphragm or membrane separating the cathode from the anode. However, sulfate and chloride impurities are also very undesirable for HAN used in propellants because these impurities can cause corrosion of the gun barrel and of components of the breech mechanism in weapons firing systems.
The hydroxylamine salt produced by the electrolytic processes of the prior art can be converted to hydroxylamine nitrate at low solution strength and in an impure state. One method is by electrodialysis as taught by Y. Chang and H. P. Gregor in Ind. Eng. Chem. Process Des. Dev. 20, 361-366 (1981). The double displacement reaction employed requires an electrochemical cell that has a plurality of compartments and requires both anion exchange and cation exchange membranes or bipolar membranes. This design entails significant capital costs and high energy costs.
U.S. Pat. No. 4,849,073 which issued Jul. 18, 1989 and is assigned to the assignee of the present invention, discloses a process and electrochemical cell to directly produce a concentrated hydroxylamine nitrate solution. A mercury cathode is used on top of a conductive plate that is also the top of the cooling compartment. This design entails the use of additional space for the separate cooling compartment and does not provide for high circulating catholyte flow rates or against the possible loss of the mercury cathode from the cell. The need to employ special precautions to prevent human exposure to mercury and the potential for contamination of the HAN with mercury are disadvantages of the electrochemical process. This process also requires more than two moles of high purity nitric acid for each mole of HAN produced, as well as a separate step to remove excess nitric acid before the HAN can be concentrated for use in propellants. Also, HAN Produced electrolytically is limited to about 3.5 molar concentration.
These and other problems are solved in the process of the present invention whereby a highly purified form of highly concentrated HAN can be Produced without excess nitric acid and with minimum HAN decomposition.